1. Field of the Invention
The present invention relates to a hologram recording material and a hologram recording method, which can be applied to a high-density optical recording medium, a three-dimensional display, a holographic optical device and the like.
2. Background Art
The general principle regarding the production of a hologram is described in some publications and technical books, for example, in Junpei Tsujiuchi (compiler), Holographic Display, Chap. 2, Sangyo Tosho. According to these publications or books, one of two beams of coherent laser light is irradiated on an object to be recorded and at the position capable of receiving the entire light reflected therefrom, a photosensitive hologram recording material is disposed. On the hologram recording material, in addition to the light reflected from the object, another coherent light beam is directly irradiated without colliding to the object. The light reflected from the object is called an object light and the light directly irradiated on the recording material is called a reference light. Interference fringes formed by the reference light and the object light are recorded as the image information. Thereafter, when a light (reproductive illumination light) same as the reference light is irradiated on the processed recording material, the light is diffracted by the hologram to reconstruct the wavefront of the reflected light first reached to the recording material from the object at the recording, as a result, almost the same object image as the actual image of the object can be three-dimensionally observed.
A hologram formed by injecting the reference light and the object light into the hologram recording material from the same direction is called a transmission hologram, where interference fringes perpendicular or nearly perpendicular to the film surface direction of the recording material are formed with a spacing of about 1,000 to 3,000 fringes per 1 mm.
Also, a hologram formed by injecting these light beams from opposite sides to each other of the hologram recording material is generally called a reflection hologram, where interference fringes parallel or nearly parallel to the film surface direction of the recording material are formed with a spacing of about 3,000 to 7,000 fringes per 1 mm.
The transmission hologram can be produced by a known method disclosed, for example, in JP-A-6-43634 (the term “JP-A” as used herein means an “unexamined published Japanese patent application) (Patent Document 1), and the reflection hologram is produced by a known method disclosed, for example, in JP-A-2-3082 (Patent Document 2) and JP-A-3-50588 (Patent Document 3).
A hologram having a sufficiently large film thickness for the interference fringe spacing (this thickness usually indicates a film thickness of about 5 times or more the interference fringe spacing, or about 1 μm or more) is called a volume hologram.
On the other hand, a hologram having a film thickness of about 5 times or less the interference fringe spacing or about 1 μm or less is called a plane hologram or a surface hologram.
Furthermore, a hologram where interference fringes are recorded by the absorption of dye or silver is called an amplitude hologram, and a hologram where interference fringes are recorded by surface relief or refractive index modulation is called a phase hologram. The amplitude hologram is greatly decreased in the light diffraction or reflection efficiency due to absorption of light and is not preferred in view of usability of light. Usually, a phase hologram is preferably used.
The hologram can reproduce a three-dimensional stereoscopic image and because of its excellent design property and decorative effect, this has been heretofore used, for example, for a cover of book, magazine or the like, a display such as POP, and a gift. In particular, for the purpose of anticounterfeit security, the hologram is used for a credit card, a bank bill, a package and the like and a market for the hologram is large at present.
The hologram for these uses is a plane-type surface-relief phase hologram and this is also called an embossed hologram because this hologram is usually mass-replicated after forming an emboss pattern from a master produced by using a photoresist.
However, the surface-relief phase hologram can hardly achieve full color formation, white color reproduction, high resolution and high diffraction efficiency and in recent years, a volume phase hologram capable of realizing these is attracting attention.
In the volume phase hologram, a large number of interference fringes having a difference in the refractive index but not in the optical absorption are formed in a hologram recording material, whereby the light phase can be modulated without absorbing light.
Particularly, a reflection-type volume phase hologram is also called a Lippmann hologram and by virtue of wavelength-selective reflection due to Bragg diffraction, this hologram can realize full color formation, white reproduction and high resolution with high diffraction efficiency and can provide a high-resolution full-color three-dimensional display.
Also, in recent years, by making good use of its wavelength-selective reflection, this hologram is widely used in practice for hologram optical devices as represented by on-vehicle head-up display (HUD), pickup lens for optical disk, head mount display, color filter for liquid crystal, reflection-type liquid crystal reflector and the like.
Other than these, studies are being made on its practical use for or application to a lens, a diffraction grating, an interference filter, a coupler for optical fiber, a polariscope for facsimile, a window glass for building, and the like.
As for known materials for recording a volume phase hologram, the write-once type includes a gelatin bichromate system, a bleached silver halide system and a photopolymer system, and the rewritable type includes a photorefractive system and a photochromic polymer system.
However, these known materials for recording a volume phase hologram are failing in satisfying all requirements required particularly in use for a high-sensitivity high-resolution full-color three-dimensional display, and improvements are demanded.
More specifically, for example, the gelatin bichromate system has high diffraction efficiency and low noise property but despite such advantages, this system has a problem in that the storability is very bad, a wet processing is necessary and the sensitivity is low.
The bleached silver halide system has high sensitivity but despite such an advantage, this system has a problem in that a wet processing is necessary, the bleaching process is cumbersome and the light fastness is poor.
The photorefractive material is rewritable but despite such an advantage, this material has a problem in that a high electric field must be applied at the recording and the recording preservability is bad.
The photochromic polymer system as represented by an azobenzene polymer material and the like is rewritable but despite such an advantage, this system has a problem in that the sensitivity is very low and the recording preservability is bad.
Under these circumstances, the dry-processing photopolymer system disclosed in Patent Documents 1 to 3 supra can be said to be a relatively practicable system capable of achieving both high diffraction efficiency and dry processing, because in this system, a basic composition comprising a binder, a radical-polymerizable monomer and a photoinitiator is used and for enhancing the refractive index modulation, an aromatic or chlorine- or bromine-containing compound is contained in either the binder or the radical-polymerizable monomer to impart a refractive index difference, as a result, the polymerization proceeds while gathering the monomer in the bright part of the interference fringes formed upon holographic exposure and gathering the binder in the dark part, whereby a refractive index difference can be formed.
However, this system has a problem in that the sensitivity is about 1/1,000 as compared with the bleached silver halide system, a heat-fixing processing as long as nearly 2 hours is necessary for enhancing the diffraction efficiency and because of radical polymerization, the diffraction wavelength and angle at the reproduction are changed due to the effect of polymerization inhibition by oxygen or resulting from shrinkage of the recording material after exposure and fixing. Thus, more improvements are demanded.
With recent progress in advanced information society, network (e.g., Internet) and high-vision TV are rapidly spreading. In addition, the start of HDTV (High Definition Television) broadcasting is near at hand. To cope with such a tendency, demands for a high-density recording medium capable of easily and inexpensively recording image information of 100 GB or more are increasing also in civilian uses.
Furthermore, along with a movement to high-capacity computers and the like, in business uses such as use for backup of computer or broadcast, an ultrahigh-density recording medium capable of recording large-volume information of about 1 TB or more at high speed and low cost is demanded.
To satisfy such demands, instead of a magnetic tape medium incapable of random access or a hard disk incapable of commutation and liable to break down, a compact and inexpensive optical medium capable of commutation and random access is attracting more attention. However, in view of physical principle, existing two-dimensional optical mediums such as DVD-R can have a capacity of about 25 GB at most even if the recording/reproducing wavelength is shifted to a short wavelength, and a recording capacity large enough to cope with the requirement in future cannot be expected.
Therefore, a three-dimensional optical recording medium is being taken notice of as an ultimate ultrahigh-density recording medium. The effective method therefor includes a method of using a two-photon absorbing material and a method of using holography (interference), and in recent years, the material for recording a volume phase hologram is abruptly gathering attention as a three-dimensional optical recording medium.
In the optical recording medium using a material for recording a volume phase hologram, a large amount of two-dimensional digital information (called signal light) is recorded by using a spatial light modulation device (SLM) such as DMD and LCD in place of the object light reflected from a three-dimensional objective. At the recording, multiplex recording such as angle multiplex, phase multiplex, wavelength multiplex and shift multiplex is performed and therefore, high-volume information reaching even 1 TB can be recorded. In the reading out thereof, CCD, CMOS or the like is usually used and by its parallel writing and reading, a high-speed transfer reaching even 1 Gbps can be attained.
The requirements required of the hologram recording material for use in a holographic memory are severer than in use for a three-dimensional display or HOE and these are:
(1) high sensitivity,
(2) high resolution,
(3) high diffraction efficiency of hologram,
(4) dry and rapid processing at the recording,
(5) capability of multiplex recording (wide dynamic range)
(6) small shrinkage percentage after recording, and
(7) good preservability of hologram.
Among these, (1) high sensitivity is a property chemically conflicting with (3) high diffraction efficiency, (4) dry processing, (6) small shrinkage percentage after recording and (7) good preservability, and it is very difficult to attain high sensitivity at the same time with other properties.
For example, the bleached silver halide system has high sensitivity but since a wet processing is necessary, this system is generally not suitable for use in a high-density recording material.
WO97/44365A1 (Patent Document 4) discloses a rewritable hologram recording material using refractive index anisotropy and orientation control of an azobenzene polymer (photochromic polymer), but this material has a problem in that the quantum yield of azobenzene isomerization is low, the sensitivity is very low because of a system accompanied with change of orientation and the recording preservability conflicting with rewritability is bad. Thus, its practical use is hardly expected.
On the other hand, the dry photopolymer system using radical polymerization described in Patent documents 1 to 3 supra has relatively high sensitivity among photopolymer systems, but the shrinkage percentage is very large and use for a holographic memory is impossible. Moreover, since the film is soft, this system is not satisfied also in the preservability.
In contrast with radical polymerization, the cationic polymerization, particularly, cationic polymerization accompanied with ring opening of an epoxy compound or the like causes less shrinkage after polymerization, is free of polymerization inhibition by oxygen and gives a rigid film. Accordingly, the cationic polymerization is sometimes said to be more suitable for holographic memory use.
For example, JP-A-5-107999 (Patent Document 5) and JP-A-8-16078 (Patent Document 6) are disclosing a hologram recording material using a cationic polymerizable compound (monomer or oligomer) in place of a binder and combining it with a sensitizing dye, a radical polymerization initiator, a cationic polymerization initiator and a radical polymerizable compound.
Also, for example, JP-T-2001-523842 (the term (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”) (Patent document 7) and JP-T-11-512847 (Patent document 8) are disclosing a hologram recording material not using radial polymerization but using only a sensitizing dye, a cationic polymerization initiator, a cationic polymerizable compound and a binder.
These cationic polymerization systems are improved in the shrinkage percentage as compared with the radical polymerization system, but on the other hand, the sensitivity is decreased and in practical use, this is considered to cause a serious problem in view of transfer speed. Also, the diffraction efficiency is decreased and this is considered to cause a problem in view of S/N ratio or multiplex recording.
As described above, the photopolymer system is a system accompanied with mass transfer and when studied on its application to a holographic memory, this encounters a dilemma that the design for good preservability and small shrinkage causes reduction of sensitivity (cationic polymerization system) and on the contrary, the design for high sensitivity causes worsening of preservability and shrinkage property (radical polymerization system). Also, multiplex recording over 50 times, preferably 100 times or more, is essential for enhancing the recording density of a holographic memory, but since the recording of the photopolymer system uses polymerization accompanied with mass transfer, the recording speed of multiplex recording in the later stage after polymerization of many compounds has proceeded decreases as compared with the recording speed of multiplex recording in the initial stage. Thus, it is a large problem in practice to control the exposure amount for preventing the reduction of recording speed and take a wide dynamic range.
On the other hand, the silver halide system forms, similarly to silver salt photograph, a latent image with slight light at the holographic exposure and the latent image is amplified at the development, therefore, its greatest attractiveness is that the sensitivity is high as compared with other systems such as photopolymer system.
However, the silver halide system which is a system of converting silver halide into developed silver (black) at the development has a problem in that, for example, the processing is wet processing and since this is an amplification type but not a phase type, the diffraction efficiency is very low. Because of these problems, this system is not suitable for holographic memory use.
The bleached silver halide system of bleaching (oxidizing) developed silver to return it to silver halide turns into a phase type and the diffraction efficiency is improved, but there arise new problems that the preservability changes for the worse, large scattering occurs and the processing is wet processing and is cumbersome. Thus, this system is also not suitable for holographic memory use.
In view of physical law, the conflict of high sensitivity with good preservability, low shrinkage percentage and dry processing, and those problems in the multiplex recording properties are unavoidable as long as conventional photopolymer systems are used. Also, the silver halide system in principle can hardly satisfy the requirements required of the holographic memory particularly in view of dry processing.
For applying the hologram recording material to a holographic memory, it is strongly demanded to develop an utterly new recording system capable of drastically overcoming these problems, particularly, satisfying high sensitivity and at the same time, satisfying low shrinkage, good preservability, dry processing and multiplex recording properties.